The present invention relates generally to implantable medical devices and more particularly to a system and method for discriminating cardiac rhythms occurring in an antegrade direction from cardiac rhythms occurring in a retrograde direction.
The heart is generally divided into two chambers, the atrial chamber and the ventricular chamber. As the heart beats, the atrial chamber and the ventricular chamber of the heart go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells are contracting to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chamber to fill the ventricular chamber. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.
Control over the timing and order of the atrial and ventricular contractions during the cardiac cycle is critical for the heart to pump blood efficiently. Efficient pumping action of the heart requires precise coordination of the contraction of individual cardiac muscle cells. Contraction of each cell is triggered when an electrical excitatory impulse (an xe2x80x9caction potentialxe2x80x9d) sweeps over the heart. Proper coordination of the contractual activity of the individual cardiac muscle cells is achieved primarily by the conduction of the action potential from one cell to the next by gap junctions that connect all cells of the heart into a functional system. In addition, muscle cells in certain areas of the heart are specifically adapted to control the frequency of cardiac excitation, the pathway of conduction and the rate of impulse propagation through various regions of the heart. The major components of this specialized excitation and conduction system include the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and specialized cells called Purkinje fibers.
The SA node is located at the junction of the superior vena cava and the right atrium. Specialized atrium muscle cells of the SA node spontaneously generate action potentials which are then propagated through the rest of the heart to cause cardiac contraction. This SA node region normally acts as the intrinsic cardiac pacemaker. The action potential generated by the SA node spreads through the atrial wall, causing the atrial chambers to contract and the P-wave of an electrocardiogram signal.
The AV node consists of small, specialized cells located in the lower portion of the atrial chamber. The AV node acts like a bridge for the action potential to cross over into the ventricular chamber of the heart. Once the action potential has crossed over to the ventricular chambers, the bundle of His carries the action potential to specialized cardiac fibers called Purkinje fibers. The Purkinje fibers then distribute the action potential throughout the ventricular chamber of the heart. This results in rapid, very nearly simultaneous excitation of all ventricular muscle cells. The conduction of the action potential through the AV node and into the ventricular chambers creates the QRS-complex of an electrogram signal.
During the cardiac cycle, the action potential moves in an antegrade direction, first causing the atrial chambers to contract and then causing the ventricle chambers to contract. When the action potential causes a single atrial contraction followed by a single ventricular contraction the heart is displaying a one-to-one atrial to ventricular response. In other words, for a given atrial contraction, the cardiac signal causing the atrial contraction subsequently causes a ventricle contraction. In this manner, there is a one-to-one atrial to ventricular response. Cardiac conditions also exist where the action potential moves in a retrograde direction, where the cardiac signal moves from the ventricular chamber up into the atrial chamber.
When a patient""s heart rate increases to above 100 beats per minute, the patient is said to be experiencing a tachyarrhythmia. Many different types of tachyarrhythmias can exist. For example, a heart in a sinus tachycardia (heart rates between 100-180 beats per minute) exhibits a normal cardiac cycle, where action potential moves in the antegrade direction from the atrial chambers to the ventricular chambers to cause the contraction of the heart. The increased heart rate during the sinus tachycardia is a response to a stimulus, and not to a cause within the heart. For example, sinus tachycardia stimulus can include physiologic responses to maintain adequate cardiac output and tissue oxygenation during exercise. Unlike sinus tachycardia, a ventricular tachycardia (heart rates between 120-250) is caused by electrical disturbances within the heart, and not due to the physiological demands of the body. Ventricular tachycardias must be treated quickly in order to prevent the tachycardia from degrading into a life threatening ventricular fibrillation.
Distinguishing a ventricular tachycardia from a sinus tachycardia is important for diagnosing and properly treating the patient""s cardiac condition. Misdiagnosis of a sinus tachycardia as a ventricular tachycardia can lead to inappropriate treatment. Difficulty in distinguishing among tachyarrhythmias increases when the heart is displaying a one-to-one atrial to ventricular rhythm. One reason for this difficulty is that the action potentials generated during the tachyarrhythmia can travel either in the antegrade direction, from the atria to the ventricles, or in a retrograde direction, from the ventricles into the atria. Tachyarrhythmias having action potentials conducted in an antegrade direction include sinus tachycardia and atrial tachycardia. Tachyarrhythmias having action potentials conducted in a retrograde direction include ventricular tachycardia with 1-to-1 retrograde conduction. Distinguishing the direction of the action potential (antegrade or retrograde) during a tachyarrhythmia is important in diagnosing and delivering the appropriate type of treatment to the patient.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a reliable and convenient approach which can distinguish antegrade and retrograde action potentials during a tachyarrhythmia.
The present subject matter provides a system and a method for distinguishing antegrade from retrograde action potentials during a tachyarrhythmia episode. The classified action potentials are then used to classify the tachyarrhythmia episode as occurring in either a retrograde direction or an antegrade direction.
In one embodiment, both atrial and ventricular contractions are detected in sensed cardiac signals. The detected atrial and ventricular contractions are then used to determine atrial and ventricular cycle lengths. In one embodiment, an atrial cycle length is the time between successively sensed atrial contractions, and a ventricular cycle length is the time between successively sensed ventricular contractions. The ventricular contractions are further analyzed to determine the occurrence of a tachycardia episode.
When a tachycardia episode is detected, the sensed atrial and ventricular contractions are analyzed to determine if the contractions have a one-to-one association of atrial contractions to ventricular contractions. During a tachycardia episode having a one-to-one association of atrial contractions to ventricular contractions, the atrial cycle lengths are paired with the ventricular cycle lengths. In one embodiment, the atrial cycle lengths are paired with the ventricular cycle lengths in the antegrade direction. In pairing the cycle lengths in the antegrade direction, an atrial cycle length is paired with a ventricular cycle length that starts just after the start of the atrial cycle length. In pairing the cycle lengths in the retrograde direction, the atrial cycle length is paired with the ventricular cycle length that starts just before the start of the atrial cycle length.
In one embodiment, as the atrial and ventricular cycle lengths are paired in the antegrade and the retrograde directions the difference of the paired cycle lengths is calculated. The differences of the paired cycle lengths are then used to calculate correlation coefficients for both the cycle lengths paired in the antegrade direction and the retrograde direction. Based on the value of the correlation coefficients, the tachycardiac episode is then classified based on the values of the antegrade correlation coefficient and the retrograde correlation coefficient.
In one embodiment, the tachycardia episode is classified as occurring in a retrograde direction when the retrograde correlation coefficient is greater then the antegrade correlation coefficient. Alternatively, the tachycardia episode is classified as occurring in an antegrade direction when the antegrade correlation coefficient is greater then the retrograde correlation coefficient.
In addition to pairing atrial and ventricular cycle lengths in the antegrade and retrograde directions, the atrial and ventricular cycle lengths can also be paired in a super-antegrade and a super-retrograde direction. In one embodiment, atrial and ventricular cycle lengths are paired in the super-antegrade direction when an atrial cycle length is paired with a first ventricular cycle length started from an action potential causing the second atrial contraction of the atrial cycle length. The atrial and ventricular cycle lengths are paired in the super-retrograde direction when an atrial cycle length is paired with a second ventricular cycle length, the second ventricular cycle length having ended before the first atrial contraction of the atrial cycle length.
As the atrial and ventricular cycle lengths are paired in the super-antegrade and the super-retrograde directions the difference of the paired cycle lengths is calculated. The differences of the paired cycle lengths are then used to calculate correlation coefficients for both the cycle lengths paired in the super-antegrade direction and the super-retrograde direction. Based on the value of the correlation coefficients, the tachycardiac episode is then classified based on the values of the super-antegrade correlation coefficient and the super-retrograde correlation coefficient. Alternatively, the tachycardiac episode is then classified based on the values of the antegrade, super-antegrade, retrograde and super-retrograde correlation coefficients.
In one embodiment, the tachycardia episode is classified as occurring in a retrograde direction when the super-retrograde correlation coefficient is greater then the super-antegrade correlation coefficient. Alternatively, the tachycardia episode is classified as occurring in an antegrade direction when the super-antegrade correlation coefficient is greater then the super-retrograde correlation coefficient.